linux-kernel - Re: [PATCH 2/2] mm: memcg: add a new MEMCG_UPDATE

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Date:   Wed, 6 Jan 2021 03:43:36 +0000
From:   Chris Down <chris@...isdown.name>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, andi.kleen@...el.com,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Huang Ying <ying.huang@...el.com>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH 2/2] mm: memcg: add a new MEMCG_UPDATE_BATCH

Feng Tang writes:
>One further thought is, there are quite some "BATCH" number in
>kernel for perf-cpu/global data updating, maybe we can add a
>global flag 'sysctl_need_accurate_stats' for
>	if (sysctl_need_accurate_stats)
>		batch = SMALLER_BATCH
>	else
>		batch = BIGGER_BATCH

Moving decisions like this to the system administrator is not really a solution 
to the problem -- inclusion should at least be contingent on either having 
"correct-ish" stats exported to userspace. Displaying broken stats to the user 
-- even with a configuration knob -- is less than ideal and is likely to 
confuse and confound issues in future.

I would also like to see numbers from more real-world workloads.  
MEMCG_CHARGE_BATCH is certainly fairly arbitrary as-is, but if it is going to 
be changed, the reason for that change and its implications (positive and 
negative) for real-world workloads must be well understood, and I'm not sure 
we're there yet.